Currently available implantable ventricular defibrillators, including the multi-programmable, pacemaker/cardioverter/defibrillator, typically employ epicardial or subcutaneous patch electrodes, alone, or in conjunction with one or more endocardial leads with one or more electrodes disposed within a heart chamber or blood vessel. Multi-lead and multi-electrode atrial and/or ventricular defibrillation systems are widely disclosed, as exemplified in U.S. Pat. Nos. 4,708,145 to Tacker, et al., 4,998,975 to Cohen et al., 5,007,436 to Smits, 5,099,838 to Bardy, 5,107,834 to Ideker et al, 5,111,811 to Smits, 5,165,403 to Mehra, and 5,174,288 to Bardy et al. Ventricular defibrillation is typically effected with at least one electrode disposed within the right ventricle and one or more electrodes disposed outside the right ventricle. Many versions of right ventricular defibrillation electrodes have been disclosed in the above listed patents and in further single endocardial lead systems as shown, for example, in further U.S. Pat. Nos. 4,481,953 to Gold et al., 4,161,952 to Kinney, et al., 4,934,049 to Kiekhafer et al., 5,010,894 to Edhag, 5,042,143 to Holleman, et al., 5,050,601 to Kupersmith et al., 5,133,365 to Heil, Jr. et al., and 5,144,960 to Mehra et al.
The positioning of the right ventricular (RV) lead electrode in proximity to the septum of the heart is considered to be desirable. In the above-referenced '975, '436, '811, '834 and '894 patents, U-shaped RV defibrillation leads are described and depicted which have at least a distal portion of the elongated defibrillation electrode shaped to bear against the septum. A U-shaped loop biased into the apex of the right ventricle is relied on to provide the force to press the distal portion back proximally along the septum. These RV leads either have a single or multiple defibrillation electrodes spaced along the lead body. In the '834 patent, the distal end of the lead bears a separate electrode that is intended to be directed into the outflow tract through the tri-cuspid valve while more proximally located RV electrodes and superior vena cava/right atrial (SVC/RA) electrodes are positioned in the right ventricle and the right atrium or superior vena cava, respectively.
In leads of this type, it is necessary to employ a separate pace/sense RV electrode bearing lead that is wedged deep into the apex of the right ventricle or to rely on a ring shaped electrode as shown in the '834 patent. In the latter case, it is not possible to obtain the deep apical penetration of the pace/sense RV ring electrode, and pacing is compromised by the poor contact with myocardial cells. In the '894 and '975 patents, additional RV lead structures are disclosed that employ active or passive fixations mechanisms for fixing the RV pace sense electrodes on the lead bodies. A pair of RV defibrillation electrodes are formed on bifurcations of the lead body or in free legs that extend back from the point of attachment in the apex of the right ventricle. These configurations are unduly complex and difficult to fabricate.
In these configurations and in the above-referenced '365 patent, attempts are made to provide at least some contact of the RV defibrillation electrodes along the septum in order to attain a distribution of defibrillation energy therein. Further approaches to distributing the defibrillation energy along and into the septum of the right ventricle are also disclosed in the above-referenced '960 and '601 patents. The RV leads disclosed therein have commonly connected branch RV defibrillation electrodes extending laterally from an intermediate or proximal point along the primary, straight RV defibrillation electrode. In the '960 patent, the branch defibrillation electrode is extended toward the outflow tract.
Despite these improvements, a problem of maintaining good contact of the elongated RV defibrillation electrode along the septum with an RV defibrillation lead that is simple to fabricate and simple to insert and position remains unsatisfied.